The invention relates to a braking system for a transport car for transporting files or goods of similar weight running on profile rails with driving rollers, equipped with a drive motor fed via wiper contacts from current paths designed on the profile rails, said braking system comprising a brake disc mounted on the motor shaft and secured against rotation relative thereto, and a counter disc aligned with the brake disc, the counter disc being withdrawn from engagement with the brake disc by means of an excited electromagnet against the force of a spring device during travel of the transport car.
The use of a drive motor for propelling a transport car along profile rails by means of driving rollers, where the drive motor receives energization via wiper contacts from current paths associated with the profile rails, is well known in the art. In this respect, reference might be made to the second and third FIGS. of U.S. Pat. No. 3,636,883 issued Jan. 25, 1972.
Such braking systems are generally provided as a supplement for short circuit braking systems of transport cars. A short circuit braking of the transport cars is to be viewed as expedient insofar as electrically driven vehicles with standard d.c. drive are accelerated by force of gravity upon downward travel on inclined rail sections in such manner that the d.c. drive changes into generator operation and, thus, the operating voltage adjacent to the current paths is increased. When a transport car is to be brought to a stop, then the short circuit braking is employed together with a switching off of the operating voltage adjacent to the current paths. In order to securely arrest the transport car in the stop position in vertical sections of the profile rails, a transport car thereby decelerated by means of short circuit braking until it stops, or respectively, until it nearly stops is arrested by means of a mechanical parking brake.
The counter disc employed in the framework of such an initially defined braking system is pressed against the brake disc by means of the spring device when the excitation of the electromagnet is switched off, whereby the spring device is arranged and dimensioned in such manner that it guarantees the immobilization of a transport car with maximum loading on the vertical profile rail section by means of the friction exerted on the brake disc. When a succeeding travel of the transport car is initiated, the electromagnet is excited and withdraws the counter disc from the brake disc. Thereby, the electromagnet must exhibit a greater attraction on the counter disc than the opposing force of the springs in the pressing position of the counter disc.
As is known, the force exerted by a magnet on a magnetizable body decreases exponentially with the size of the air gap between the magnet and this body. Tolerances conditioned by fabrication and operation, therefore, have a significant influence on the dimensioning of the electromagnet.